1. Field of the Invention
The present invention concerns a method to generate magnetic resonance (MR) images in a volume of interest of an examination subject, and a method to generate an MR image of a first excitation volume of the examination subject, as well as an MR system to implement such methods.
2. Description of the Prior Art
The optimization of the image quality represents one of the central themes in the development of tomographic imaging modalities such as magnetic resonance (MR) systems. In a typical examination workflow, an overview image (known as a localizer image or simply “localizer”) is initially acquired in which the actual diagnostic measurements for the generation of MR images are then planned, with which MR images a physician wishes to answer a desired medical question. In addition to the selection of suitable contrast parameters such as the sequence type or sequence parameters (such as echo time TE, repetition time TR), the extent, attenuation and position of the acquisition volume is thereby established, meaning that a first excitation volume in which the magnetization is excited and that is then shown in the MR image is established. For example, this first excitation volume can encompass a number of slices in the case of a two-dimensional imaging, or can be in the form of blocks in a three-dimensional imaging.
Additional excitation volumes in which the magnetization of nuclear spins is excited (also called active volumes in the following) can result, for example, by the establishment of local saturation regions, for example for the suppression of unwanted image regions. This suppression can be desirable if movement, flow or pulsations are to be expected, for example. Additional excitation volumes are local or global preparation volumes—for example the suppression of the fat signal in the total examination volume—or local marking volumes, for example to mark incoming blood for contrast agent-free determination of flow or perfusion. The acquisition volume—i.e. the first excitation volume—often encompasses a markedly larger region than the actual volume of interest. The volume of interest is the volume in which an assessing physician wishes to obtain information to clarify a medical question. For example, the volume of interest can include a potential pathology. The reasons that the first excitation volume is larger than the volume of interest lie first in that (among other things) the possibility for fast orientation in the anatomy is desired, and second in that the aliasing artifacts should be avoided. However, such a large acquisition volume can be accompanied by a reduced image quality in the volume of interest. The control of all technical acquisition modules is optimized for the acquisition of the first excitation volume, which automatically entails a compromise for the volume of interest.
An additional disadvantage of the aforementioned planning method is the long time cost for the planning of the individual excitation volumes.
The limitation of the acquisition volume to the volume of interest is possible in principle, for example via a selective spatial excitation of this volume. However, the possibility for fast orientation in the anatomy is hereby lost. Furthermore, such selective excitations are frequently linked with additional disadvantages, for example a longer echo time and a longer acquisition time connected with this. The volume of interest is typically established manually by the user. In addition to the first excitation volume, the acquisition volume of possible additional excitation volumes are thereby established by the user in the overview image with a graphical user interface with regard to attitude, orientation and position. However, an experienced user is necessary for this; the placement of the individual volumes is poorly reproducible and costly in terms of time. Furthermore, the automatic establishment of individual excitation volumes (called working volumes there) is known from DE 10 2009 020 661 A1.